Polymeric nanocomposite

ABSTRACT

The invention relates to a process for the preparation of a polymeric nanocomposite, comprising a polymer selected from the group comprising nylon, polyester and polyurethane, and comprising graphite. The process results in a nanocomposite, comprising 5-20 wt. % of graphite, said nanocomposite having both ESD- and FR-properties. The invention also deals with equipment, at least partially made of nanocomposite.

The present invention relates to a process for the preparation of apolymeric nanocomposite, said nanocomposite comprising a polymerselected from the group comprising nylon, polyester and polyurethane,said nanocomposite also comprising graphite.

Such a process is known from an article in Journal of Polymer Science,pages 1626-1633, 2001, and written by Yu-Xun Pan et. al. This articledeals with the preparation of a nylon/graphite nanocomposite. Thisprocess results in a nylon composite being electrically conductive. Thearticle does not only not deal with the use of such composite, it alsofails to recognize that in areas where also flame retardancy is a keyissue, such a composite does not meet the qualification for aflame/retardant product.

There are several major applications for such polymeric nanocomposite,especially in areas where reduction of friction and/or of weight isimportant ((partial) replacement of metal parts by plastic parts).

The reduction of friction and/or of weight may well result indeterioration of those properties which go along with the replaced part,especially for products obliged to have antistatic (protection fromelectrostatic discharge; ESD) as well as flame retarding (FR)properties.

The present process overcomes these deficiencies in providing aninventive process for the preparation of a polymeric composite, whichfulfils the above mentioned needs.

This goal is achieved in a process, comprising the following steps:

-   a) mixing the liquid monomer(s) for the polymer, or a liquid    oligomer thereof, with an intercalated graphite using a specific    mixing energy of at most 1 kW/m³,-   b) degassing the resulting mixture for a period of at least 5    minutes under a pressure of at most 50 kPa,-   c) polymerizing said mixture in the presence of a suitable catalyst    system, the process resulting in a polymeric nanocomposite,    comprising 5-20 wt % of delaminated graphite.

The elements of the process steps of the present invention will be dealtwith below.

a) The process starts with mixing the precursor(s) of the final polymerin liquid form with an intercallated graphite, said mixing being with areduced amount of energy, the specific mixing energy being at most 1kW/m³, more preferred at most 0.75 kW/m³. Initially the mixture isrelatively “viscous”; the mixing has to be continued until this mixturehas become homogeneous. There are several methods known in the art todetect the moment of the obtainance of a homogeneous mixture, likevisually or measuring the torque of the stirrer, used for mixing.Monomers suited for the preparation of a polymer nanocomposite, whereinthe polymer is selected from the group comprising nylon, polyester andpolyurethane, are well known in the art. As an example of a monomer fornylon can be mentioned caprolactam, for the preparation of nylon-6; theskilled man of art knows the monomer(s) to be used for in-situpolymerization to the mentioned polymeric nanocomposite. The monomer(s)can be either suited for the preparation of homopolymers as well as forthe preparation of copolymers, like an impactresistant nylon/polyetherblockcopolymer. The skilled man is aware of suitable monomers therefor.An alternative is the use of mixed monomers for a nylon/nylon mixture,like a combination of caprolactam and laurinolactam, resulting in anylon-6/nylon-12 mixture.

The monomer(s) need(s) to be in liquid form, as a result of which oftenthe monomer(s) need(s) to be brought in said form by a melt-process. Forsaid caprolactam this means that a temperature of at least 70° C. isneeded, before the caprolactam is liquid. Depending on the monomer, theskilled artisan can select the temperature at which said mixing shouldtake place.

Not only monomer(s) can be used in the present process, but also aliquid oligomer of the intended polymer can be used, again depending onthe nature of the polymer and the related oligomer. Addition of aseparate impact modifier can also be done at this stage; an example ofsuch a modifier is jeffamine, like KU2-8112 of Bayer. In general theviscosity of the monomer(s) or oligomer should not exceed 50 mPa.s. Asthe graphite to be used in the process of the present invention, anygraphite-like product is useful, where the distance between crystallinelayers of the graphite have been extended with a gas or liquid,resulting in an intercalated graphite; in the above mentioned literaturereference referred to as graphite intercalation compound (G.I.C.). ThisG.I.C. is used as such in the present invention. An example hereof isTimrex® from Timcal. As alternative also an expanded graphite (EG) canbe used; this product is obtainable by rapid heating (at temperatureswell above 250° C.) of a G.I.C., resulting in an expanded and exfoliatedgraphite. Preferably this heating results in an EG having an expansionratio of at least 150, more preferably an expansion ratio of at least200. Examples of such an E.G. are Nord-min® of Nordmann, Rassmann GmbH(http://www.plastverarbeiter.de/product/e958cf21 ccf.html) and E type ES100 C. 10 from Kropfmühl A. G. The graphite preferably has an aspectratio (=length/thickness ratio) of at least 100, more preferably atleast 150. This results in an optimal value for both FR- andESD-properties.

b) The mixture, resulting in step a) is to be degassed, during and/orafter the mixing process, in order to facilitate the intimate mixing ofthe polymer precursor(s) and the graphite. Although it is possible todegas at ambient pressure, from an economical point of view saiddegassing is done under a vacuum of at most 50 kPa, during a period ofat least 5 minutes. The lower the vacuum-pressure at the degassing step,the faster the degassing can take place. Preferably the degassing ratio(D.G.-ratio), herein defined as the ratio between the time of degassing(in minutes), and the vacuum-pressure during degassing (in kPa), is atleast 1; in formula:${D.G.{- {ratio}}} = {\frac{{time}\quad({minutes})}{{pressure}\quad({kPa})} \geq 1.}$This degassing should be performed while the precursor/graphite mixtureis in liquid form.

It has surprisingly been found that with the use of a mixture of aG.I.C. and an E.G., the desired properties of the polymericnanocomposite can even better be achieved. In doing so, one canindependently accommodate for the FR and ESD properties. A processvariant hereof is, that both the G.I.C. and the E.G. are mixed with theprecursor(s) of the final polymer, followed by step b). An alternativeis the mixing of the G.I.C. with the precursor(s), performing step b),followed by an addition of the EG to the resulting mixture, followed bya second degassing step (b)-step).

c) The degassed mixture is then polymerized in suitable equipment,optionally in the presence of a suitable catalyst system, underconditions known in the art for the polymerization to either nylon,polyester of polyurethane. As a result of the polymerization, thegraphite is substantially present in delaminated form in the polymericnanocomposite.

To achieve the goal of the present invention, the polymericnanocomposite resulting form the described process should have agraphite content of between 5 and 20 wt. %, relative to the weight ofthe polymer. When using a mixture of G.I.C. and E.G., the amount ofG.I.C. in the polymer can preferably be varied between 5 and 10 wt %;the amount of E.G. in the polymer can preferably be varied between 5-15wt %. In such a combination, both the required ESD and FR properties canbe obtained.

The intercalated graphite to be used in the process of the inventionshould have a particle size of at most 75 μm, preferred at most 25 μm,and more preferred at most 10 μm. In doing so, the effectiveness of thegraphite in the obtainance of both ESD- and FR-properties is improved.The expanded graphite has a particle size of at most 200 am; preferably80% of the particles are smaller than 150 μm.

The process of the present invention is preferably suitable for ananionic polymerization; more preferred even where this polymerization isa monocast in-mould polymerization, wherein the mixture comprisingprecursor and graphite is cast (poured) into a mould with a predesignedshape, where in said mould the polymerization is performed.

In preference, the process of the present invention results in apolymeric nanocomposite, based on a nylon, selected from the groupcomprising nylon 6, nylon 11 and nylon 12.

It has been found that the properties of the polymeric nanocomposite canbe further improved by heat-annealing the composite at elevatedtemperatures (but below the melting point of the composite); in order toreduce the amount of residual monomer(s).

The invention also relates to a polymeric nanocomposite having bothdesired FR- and ESD-properties. The nanocomposite comprises as polymericelement a polymer selected from the group comprising nylon, polyester,and polyurethane; preferably the polymer is nylon, selected from thegroup comprising nylon 6, nylon 11 and nylon 12. The melt viscosity ofthe nylon, determined at 260° C., preferably is at least 8 kPa.s, asdetermined according to ISO 6721-10. The polymeric nanocomposite of theinvention is comprising 5-20 wt. % delaminated graphite, and is having asurface resistivity of between 10⁴ and 10¹⁰ Ω/square, as well as aflame-retardancy of at least UL94V1. The surface resistivity is to bemeasured according to ASTMD257; the flame-retardancy according toUnderwriter Laboratory Test '94. Preferably, the surface resistivity isbetween 5×10⁵ and 10¹⁰ Ω/square. The FR-properties can also bedetermined according to DIN 22100-7, in which the dripping behaviour ofa specimen under fire is determined. In this test, the time for thespecimen to start dripping is determined. This time should be preferablyat least 15 minutes, more preferred at least 20 minutes, in order todesignate the product as being flame-retardant. Also preferred is aflame-retardancy of at least UL94V0.

The polymeric nanocomposite of the present invention may also compriseconventional additives and other fillers, as they are known in the artto be used in polymeric compositions comprising nylon, polyester ofpolyurethane. Such additional components can comprise coulorants,reinforcing agents, fibers of polymeric or natural nature, etc. Theskilled man of art knows which to select.

The polymeric nanocomposite of the present invention is very wellsuited, due to its ESD- and FR-properties, to be used in equipment andmaterials to be used in areas where these properties play a significantrole. Public authorities have evermore demanding requirements on suchequipment and materials, in order to prevent casualties and materialdamage in case of fire and/or electrostatic problems.

Especially in underground mining activities, and more dedicated incoalmining activities, these requirements play a significant role. Thepolymeric nanocomposite of the present invention is able to meet theserequirements and can therefor be used in such equipment and materials,which are at least partially made of said nanocomposite. In said miningactivities, and especially in said coal mining, the equipment andmaterials which are at least partially made of said composite,preferably are in the form of a flight bar, and/or of a conveyer roller.These parts are extremely sensitive for ESD- and FR-conditions. To dateheavier and/or much more expensive materials are used, which can now bereplaced, at least partially, by equipment and materials of thisinvention. The referenced equipment and materials can, in a formaccording to the present invention, be of an hybride nature, being acombination of either the polymer and fibers of metal or of polymericnature (like steel, or polyethylene fibers), or in which part of theequipment is made of metal (like steel or alumina) and the rest is madeof the above described polymeric nanocomposite. Reference can be givento a metal-in-polymer product, as well as a metal-on-polymer product.

The polymeric nanocomposite can also be used in other types of equipmentand materials, preferably in transportation elements where the FR- andESD-properties can be exploited, preferably in transportation elementsunderground or in tunnels. Without limiting to the following areas ofuse, mentioning can be made of:

-   -   use in tunnels, like plugs or railroad equipment    -   use in airports, like parts for people and luggage conveyer        escalators    -   metro and underground people transport parts for escalators    -   off shore activities, including sub-marines    -   conveyor profile covers;        in essence in all closed areas where safety for people and        materials is important; this normally being the case where there        is friction between plastic parts and/or between plastic and        metal parts. The present demands are for flame-retardancy of at        least 15 minutes after initiation of the fire.

The invention will be elucidated by means of the following Examples,which are not meant to limit the scope of the invention.

EXAMPLE I

In a 250 ml round-bottomed flask, 75 grams of caprolactam flakes (waterconcentration <100 ppm) and 5 grams of dried Timrex® KS44 graphite wereadded. The intercalated graphite had an average particle size of 44 μm.The flask was flushed with dry nitrogen and heated in an oil bath at120° C. to melt the caprolactam. The mixture was stirred using amagnetic stirrer rod at 200 rpm (specific mixing energy ca. 0.1 kW/m³)and evacuated for 6 minutes at a pressure of 500 Pa. After breaking thevacuum, 1.5 grams of activator (Brüggolen® C20. C20: caprolactam hexanedi-isocyanate prepolymer (CAS 5888-87-9)) was given to the mixture understirring at 100 rpm.

In the meanwhile 3 grams of anionic catalyst (Brüggolen® C10. C10:sodium salt of aliphatic cyclic acid amide; specifically sodium salt ofcaprolactam (CAS 2123-24-2)) was dissolved under a dry nitrogenatmosphere in 7 grams of dry caprolactam in a laboratory reaction tubeat 120° C. and homogenized by shaking.

The catalyst solution was poured to the graphite containingcaprolactam/activator mixture and the mixture was homogenized by shakingfor 5 seconds. The homogenized mixture was poured in a glass mould(diameter 40 mm), preheated in an oil bath at 140° C. In the mould at140° C., polymerization of the caprolactam and crystallization of theresulting nylon-6 occurred within 10 minutes.

After demoulding, the surface resistivity of the polymer, containing 5wt % of graphite, was 10⁹ Ω/square.

EXAMPLES II-IV

Nylon-6 samples were produced according the procedure described inExample I except for the amount and kind of intercalated graphite. Theresulting surface resistivity of the samples after demoulding was:amount surface resistivity Graphite (wt. %) (Ω/square) Timrex ® KS6 910⁷-10⁸ Timrex ® KS44 10 10⁷ Timrex ® KS6 15 10⁶

EXAMPLE V

In a 2 ltr round-bottomed flask, 640 grams of caprolactam flakes (waterconcentration <100 ppm) and 154 grams of dried Timrex® KS44 graphitewere added. The flask was flushed with dry nitrogen and heated in an oilbath at 120° C. to melt the caprolactam. The mixture was stirred at 100rpm with a blade stirrer and evacuated for 60 minutes at a pressure of30 kPa. After breaking the vacuum, 12 grams of activator (Brüggolen®C20) were given to the mixture under stirring at 100 rpm.

In a 1 liter round-bottomed flask, 17 grams of anionic catalyst(Brüggolen® C10) was dissolved at 120° C. under a dry nitrogenatmosphere in 380 grams of dry caprolactam and homogenized by stirring.

The catalyst solution was poured to the graphite containing activatorsolution and the mixture was homogenized by stirring at 100 rpm for 4seconds. The homogenized mixture was poured in a stainless steel mould(10*10*20 cm), preheated in an oven at 140° C. After 15 minutes at 140°C., the mould was opened to obtain the polymer produced.

After demoulding, the surface resistivity of the polymer, containing 17wt. % of graphite, was 10⁶ Ω/square.

For detecting the dripping behavior of the polymer produced, a flamewith a tip temperature of 900° C. was placed at 40 mm from the product(according to DIN 22100-7). After 18 minutes the polymer started todrip. Extinguishing the flame resulted also in a switch-off of theburning of the product.

EXAMPLE VI

The same procedure and amounts as described in Example V was used toproduce a sample. After demoulding, the sample was annealed at 155° C.for 24 hrs.

The result of the annealing procedure was that in the drip test,dripping started after 25 minutes.

EXAMPLE VII

A Nylon-6 sample was produced according to the procedure described inExample I, except for the amount and type of graphite: a mixture of 5 w% of Timrex® KS6 and 10 wt % of Nord-min® 35.

The surface resistivity of the resulting mould was 10⁸ Ω/square. Themoulded product showed a clear decrease of flame intensity compared toproducts only filled with intercalated graphite.

1. Process for the preparation of a polymeric nanocomposite, comprisinga polymer selected from the group consisting of nylon, polyester andpolyurethane, and comprising graphite, this process comprising thefollowing steps: a) mixing the liquid monomer(s) for the polymer, or aliquid oligomer thereof, with an intercalated graphite using a specificmixing energy of at most 1 KW/m³, b) degassing the resulting mixture fora period of at least 5 minutes under a pressure of at most 50 kPa, c)polymerizing said mixture, optionally in the presence of a suitablecatalyst system, the process resulting in a polymeric nanocompositecomprising 5-20 wt. % of delaminated graphite, relative to the weight ofthe polymer.
 2. Process according to claim 1, wherein a mixture of theintercalated graphite and an expanded graphite is used.
 3. Processaccording to claim 1, wherein the particle size of the intercalatedgraphite is at most 75 μm, preferably at most 25 μm, more preferred atmost 10 μm.
 4. Process according to claim 1, wherein the expandedgraphite has a particle size of at most 200 μm; preferably 80% of theparticles are smaller than 150 μm.
 5. Process according to claim 1,wherein step c) is an anionic polymerization.
 6. Process according toclaim 5, wherein the polymerization is a monocast in-mouldpolymerization.
 7. Process according to claim 1, wherein the polymer isa nylon, selected from the group consisting of nylon 6; nylon 11; andnylon
 12. 8. Process according to claim 1, wherein the intercalatedgraphite is expanded to an expansion ratio of at least
 150. 9. Processaccording to claim 1, wherein the intercalated graphite has an aspectratio of at least
 100. 10. Polymeric nanocomposite, comprising a polymerselected from the group consisting of nylon, polyester, andpolyurethane, wherein said nanocomposite comprises 5-20 wt. %delaminated graphite relative to the weight of the polymer, and having asurface resistivity of between 5×10⁵-10¹⁰ Ω/square (according to ASTMD257), and a flame-retardancy of at least UL 94 V1 (according toUnderwriter Laboratory Test 94).
 11. Polymeric nanocomposite accordingto claim 10, wherein the polymer is a nylon, selected from the groupconsisting of nylon 6; nylon 11; and nylon 12, or mixtures. 12.Polymeric nanocomposite according to claim 11, wherein the nylon has amelt viscosity of at least 8 kPa.s.
 13. Equipment useful in areas whereelectrostatic discharge and flame-retardancy play a significant role,wherein at least part of the equipment is made of the polymericnanocomposite according to claim
 1. 14. Equipment according to claim 13,suited for use in underground mining activities.
 15. Equipment accordingto claim 14, suited for use in coal mining.
 16. Equipment according toclaim 14 in the form of a flight-bar.
 17. Equipment according to claim14 in the form of a conveyer roller.
 18. Equipment according to claim14, suited for use in transportation elements underground or in tunnels.19. Equipment according to claim 13, wherein the equipment comprises ahybride combination of the polymeric nanocomposite and either: a) apolymeric or metal fiber b) a metal in the form of either b1) ametal-in-polymer product, or b2) a metal-on-polymer product. 20.Equipment according to claim 19, wherein the metal is steel or alumina.